1. Field of the Invention
The present invention relates generally to downhole pumping systems and, more particularly to a motor protector for use with a downhole pumping system.
2. Description of the Related Art
Electric submergible pumps are widely used throughout the world for recovering subterranean fluids to the earth's surface. For the long term successful operation of such submergible pumping systems, the electric motor is supplied with uncontaminated motor oil. The motor oil not only lubricates the motor, it also cools the motor to prevent overheating.
In most submergible pumping systems in use today, this motor oil is partially contained within a device commonly referred to as a motor protector. Conventional motor protectors typically include one or more elastomeric bags. These elastomeric bags provide two important functions: (1) equalizing the fluid pressure within the motor and (2) preventing well fluids and gases from contaminating the motor oil.
In regard to the first function, it should be understood that the temperature of the motor oil varies as a result of the intermittent operation of the submergible motor. As the temperature of the motor oil rises, for instance, the oil tends to expand and the pressure within the motor tends to increase. If the motor protector did not include an expandable member, such as the elastomeric motor protector bag, the internal pressure of the motor would increase dramatically. However, the motor protector bag expands and contracts to compensate for the varying liquid volume and to maintain a relatively constant pressure within the motor.
In regard to the second function, the motor protector bag provides a degree of isolation between the motor oil and the well fluids and gases. This isolation helps keep the motor oil clean to increase the longevity of the motor. Most elastomeric motor protector bags prevent many contaminants, such as crude oil, water, brine, and dirt, which may greatly reduce the life of the motor, from entering the motor.
As discussed above, in many applications elastomeric motor protector bags perform reasonably well. However, elastomeric bags suffer from several limitations. First, the repeated expanding and contraction of the elastomeric bag can cause the bag to split or crack under certain conditions. Of course, once an elastomeric bag splits or cracks it no longer protects the motor oil from contaminants which are then free to enter and ultimately damage the motor. Second, elastomeric bags tend to lose their elasticity due to various conditions which may be present in a wellbore. Once an elastomeric bag loses its elasticity, it can no longer expand and contract as needed to satisfy the requirements of the motor oil which it contains. Eventually the bag will rupture, leaving the contaminants free to attack the motor. Third, most elastomers cannot survive in environments where the temperature rises above about 400.degree. F. Above that temperature, most elastomers become brittle causing the bag to break during expansion or contraction. Finally, elastomeric compounds currently used for motor protector bags tend to be relatively permeable as compared to the contaminants within the wellbore fluid. Many wells contain contaminants, such as hydrogen sulfide for instance, which will permeate the motor protector bag and attack the motor. In fact, certain contaminants, such as hydrogen sulfide, also tend to alter the chemistry of certain elastomers, causing the elastomers to harden. Once the elastomer has hardened, the bag eventually breaks.
In an effort to combat one or more these problems, the elastomeric material used to fabricate the motor protector bags have been studied and chosen to provide certain advantages. For instance, certain elastomers may slow the rate at which contaminants such as hydrogen sulfide enter the motor, but they cannot stop the permeation completely. Alternatively, certain elastomers may exhibit an ability to withstand temperatures as high as about 400.degree. F., but these elastomers tend to have limited elasticity incompatible with the requirements of the motor.
The present invention may address one or more of the problems discussed above.